Computer tomography (CT, also called computed tomography) has evolved into a commonly used means, when it comes to generating a three-dimensional image of the internals of an object. The three-dimensional image is created based on a large number of two-dimensional X-ray images taken around a single axis of rotation. While CT is most commonly used for medical diagnosis of the human body, it has also been found applicable for non-destructive materials testing. Detailed information regarding the basics and the application of CT, can be found in the book “Computed Tomography” by Willi A. Kalender, ISBN 3-89578-216-5.
One of the key innovative aspects in future CT and X-ray imaging is the energy-resolved counting of the photons which are let through or transmitted by the object being analyzed when being exposed to X-ray radiation. Depending on the number and energy the transmitted photons have (which have not been absorbed), it can be concluded through which type of material the X-ray radiation has traveled. In particular, this allows to identify different parts, tissues and materials within a human body. Even more specifically, the absorbing coefficient is energy dependent, thus developing a detector capable of resolving the energy of the impinging photons allows solving that dependency and leads to the elimination of “beam hardening” effects.
When the detection or counting of photons is referenced, it is understood, that when a photon impinges on the conversion material of a sensor, it creates a charge pulse (sometimes also referred to as current pulse). This charge pulse is detected and the presence of a photon is concluded. The charge pulse results from a larger number of electron-hole pairs, which are generated, when an X-ray photon interacts with the sensor conversion material. The duration of this charge pulse corresponds to the so-called charge collection time.
Detection of single electron-hole pairs is not in the focus of this application, but the processing of a charge pulse resulting from electron-hole pairs representing a photon, which may also be expressed by the formulations “detecting photons” or “counting photons”. For a charge pulse, which is generated by interaction of an X-ray photon, also the formulation is used that the charge pulse belongs to this X-ray photon. Along the same lines, e.g. “processing a charge pulse caused by a photon impinging on the sensor” is sometimes also denoted as “processing a photon” in the following.
One of the main concerns when implementing a counting detector for computer tomography applications is to deal with the non-periodic nature of the incoming photons. The flux of photons, which has to be considered, is very high and randomly distributed in time. The distribution of the photons can be described by a Poisson distribution.
Due to the random distribution so-called pile-up events are likely to occur. This means, that one or more additional photons may arrive before the previous photon has been processed by the detector. Such pile-up events can lead to incorrect results and can significantly reduce the energy resolution of the imaging device.
The time window in which a new event cannot be detected, meaning that a new impinging photon cannot be processed, is called the dead time of the detector. When correctly considering the dead time of the detector, pile-up events can be better considered. However, there always remains the risk, that events/photons are only partly processed and/or that events/photons are disregarded.
One concept of how to address the rejection of pile-up pulses is described in the application note “A Practical Guide to High Count Rate Germanium Gamma Spectroscopy”, by Can berra Industries, Inc.; no. NAN 0013, August 1993, pages 10-12.